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PODjango: How do I count the number of objects related to another in the database?
 

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  1. PODjango: How do I count the number of objects related to another in the database?
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    <p>I want to make an application that will allow people to form groups to buy items together. I need a way to see how many people are already buying. I have the following models.py:</p> <pre><code>from django.db import models class Buyer(models.Model): ''' Buyer of the item ''' first_name = models.CharField(max_length=40) last_name = models.CharField(max_length=40) email = models.EmailField() # Pieces together full name from first and last def full_name(self): return "%s %s" % self.first_name, self.last_name # String representation of buyer in the format 'firstname lastname (email)' def __unicode__(self): return "%s (%s)" % self.full_name, self.email class Item(models.Model): '''The item being purchased''' name = models.CharField(max_length=50) description = models.TextField() price = models.DecimalField(max_digits=6, decimal_places=2) # The number of items to be split quantity = models.IntegerField() pub_date = models.DateTimeField('Date Offered') # Expiration of the item on offer expires = models.DateField() # Relates item to buyers buyers = models.ManyToManyField(Buyer) # Calculates remaining slots def remaining(self): # How do I do this? # String representation of item using item name def __unicode__(self): return self.name </code></pre> <p>I know conceptually that I simply need to take the quantity in the offer and subtract the number of buyers, but I don't really know how to accomplish this. Based on what I've read, it seems I may be able to do it with something like this:</p> <pre><code>def remaining(self): return self.objects.aggregate(Count('buyers')) </code></pre> <p>I'm not very confident, though. Can anyone help me out with the best approach?</p>
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